"We are worried about chemical weapons," said Mr Hague. "We have become more concerned about them in recent days for the same reasons the US has. We have already sent our own, clear, private message directly to the Syrians about the serious consequences that would follow from the use of those weapons."

Britain conveyed its message through diplomatic channels, but officials declined to go into details. The US administration is believed to have satellite images showing the movement of essential materials for chemical weapons.

The Assad regime is known to possess sarin nerve gas, which is made from four or five "precursor" chemicals which would normally be kept in separate installations. Western governments will be looking for evidence of these materials being moved into a single facility, said Dina Esfandiary, an expert on Syria's weapons of mass destruction at the International Institute for Strategic Studies.

If the precursors were brought together in the same plant, the army would be preparing sarin gas for use, said Ms Esfandiary, adding: "There really isn't another plausible explanation".

Riad Kahwaji, the head of the Institute for Near East and Gulf Military Analysis, said that suspicious movements had been detected in the last few days. "We are dealing with a regime that has not hesitated to use the heaviest weapons it has. It lives in denial, still thinking that it can reverse the clock and has threatened everyone with starting a regional war. It has to be taken very seriously when it we see that is moving around chemical weapons," he added.

But a western diplomat in the region said: "On the question of chemical weapons in Syria and whether the regime would use them I think the correct response is caution caution caution. There are so many unknowns, so little information available that it becomes almost impossible to know what the regime may or may not be doing." The diplomat added: "My gut is that we are not there yet. They would likely use them only if they are really really desperate and I don't think this is yet the case".

Turkey has asked Nato to deploy batteries of Patriot missiles to protect its southern border with Syria. These advanced weapons are designed to shoot down aircraft or missiles. Turkey's request is believed to have come after intelligence assessments concluded that Mr Assad's regime, now under severe pressure from the rebel Free Syrian Army which has penetrated Damascus itself, could resort to firing its arsenal of Scud ballistic missiles at neighbouring states. In extremis, these weapons could be equipped with chemical warheads.

"We know that Syria possesses missiles. We know they have chemical weapons and this is also the reason why it is necessary to ensure effective defence and protection of our ally Turkey," said Anders Fogh Rasmussen, the Nato secretary-general.

Mr Hague made clear that Britain backed the deployment of Patriots to "show solidarity with Turkey". He added that such a deployment would also "send an important message to the Syrian regime".

But Nato officials stressed the defensive purpose of deploying the Patriots, stressing they would not be used to enforce a no-fly zone over Syrian territory. "The purpose is to protect the Turkish population and territory against missile attacks," said Mr Rasmussen. "Any deployment would be defensive only. It would in no way support a no-fly zone or any offensive operation."

The Patriot missiles will be supplied by Germany, Holland and the US. About six batteries, protected by between 300 and 400 troops, would be deployed in the New Year. The "practical issues" are expected to be settled "within weeks", said Mr Rasmussen.

But Russia, which serves as Syria's staunchest international ally, warned against the deployment. "Creating additional capabilities on the border does not defuse the situation but on the contrary exacerbates it," said President Vladimir Putin.

Separately, 29 students and a teacher were reportedly killed near Damascus on Tuesday, according to Syrian state media, after a rebel mortal hit a classroom in a displacement camp.